Cable television distribution arrangements are well known. In such arrangements, analog television signals are carried to customers via a branched coaxial cable which includes bridger amplifiers, line extenders, and customer taps. Each television signal occupies a 6 MHz channel at a frequency from about 50 MHz to about 550 MHz or more. The upper frequency is limited by the bandwidth of the bridger amplifiers and line extenders and the attenuation of the coaxial cable, which as is well known increases with increasing frequency.
There is an increasing desire for additional capacity in cable television distribution arrangements. This demand includes a desire for additional broadcast television signals in analog or compressed digital form, additional facilities including for example video-on-demand (VOD) and near-VOD services (e.g. movies broadcast with stepped starting times), and a desire for transmission of control and possibly other information in the opposite, upstream, direction via the network.
To meet this desire, various ways have been proposed for supplying additional signals via a cable television distribution arrangement, typically involving the supply of such signals via optical fibers to appropriate points in the coaxial cable system with delivery of the signals to the customer premises via the coaxial cable, referred to as the drop cable, which already exists from the customer tap to the customer premises.
It is desired to supply to each customer only those services or television signals for which the customer has agreed to pay. As different customers have different preferences, there is a need to provide, at each customer tap, equipment which extracts from all of the signals which are available on the network only those which are allocated for each particular customer. Only these signals are then delivered in modulated form for transmission to the customer via the respective drop cable.
One way of doing this is to provide an individual modulator for each customer. However, this requires a large number of modulators, each of which is powered via the coaxial cable, with consequent disadvantages of high complexity, costs, and power consumption and dissipation. An alternative is to provide a modulator which is common to a group of customers; however, this falls to meet the above desire in that it delivers to all customers in the group the signals for which only one customer in that group may have agreed to pay, thus involving a risk of other customers in the group receiving signals to which they are not entitled.
Accordingly, an object of this invention is to provide an improved data modulation arrangement for selectively distributing dam.